Turning up the heat: Assessing the impact of charring regime on the morphology and stable isotopic values of cereal grains

Abstract

The stable isotopic values of charred crops are now frequently analysed in archaeology. While previous research has highlighted how grain morphology and stable carbon and nitrogen isotope values change with grain charring temperature, such research has been limited to temperature ranges under 260 °C and using predominately Mediterranean cereals and pulses. For the first time, this study provides experimental data on the impact of charring on two northern European cereals, rye and oat, both morphologically and isotopically. New experimental charring of rye, oat, bread wheat and hulled barley extends the charring window to 300 °C, providing an insight into the morphological changes to the grains as well as the difference between charred and uncharred isotopic values. This range of cereals and conditions opens up potential for stable isotopic investigation of medieval agricultural growing conditions and practices in Britain. The results indicate that isotopically, a 0.16‰ and a 0.32‰ offset should be applied to δ13C and δ15N values, respectively, of grains charred between 230 and 300 °C. Morphological and internal structural changes, as well as external distortion, are key attributes which vary with charring temperature and duration. Guidelines are provided to enable assessment of whether archaeological grains of bread wheat, hulled barley, rye and oat fall within the acceptable charring window for isotopic analysis.